How malloc works? [duplicate]

Answer 1

The question is kind of wrong.

In a typical OS, there exists the concepts of virtual memory and physical memory.

Physical memory exists typically in 4kb blocks, virtual memory likewise.

Each process has virtual memory - to each process the OS presents what appears to be the fully addressable memory range. So on a 32 bit machine, each process 'thinks' it has 4 GB of contiguous memory.

In reality, the OS, behind the scenes, is busy mapping virtual memory allocations onto real blocks of physical memory. So a, say, 400kb virtual memory allocation, is mapped onto 100 physical blocks. Those physical blocks do not need to be contigious (and almost never are - nothing stops it from happening, but on a machine doing any kind of work, it's highly improbable) but the virtual memory allocation does need to be contiguous.

So you can still run into virtual memory fragmentation. Here, a process requests a block of memory and there isn't, in that particular processes virtual memory map, a block of contiguous virtual memory such that the request can be satisfied.

That problem is the problem you're thinking of.

Answer 2

This is platform specific. To your eyes, on the software level, it will always be presented as you malloc:ed 20 contiguous bytes. But on some platforms, e.g. with paged memory, these bytes do not really need to be contiguous when it comes down to the actual hardware - it will just appear so.

Answer 3

Standard malloc is defined in the C standard to allocate a contiguous block of memory (at least it appears so to you) - it will return a null pointer if the allocation fails.

At a lower level, the OS will be doing something like what kotlinski or Blank Xavier have described in their respective answers.

From §7.20.3 of the ISO/IEC 9899-1999 C Standard:

The pointer returned if the allocation (by calloc, realloc, or malloc) succeeds is suitably aligned so that it may be assigned to a pointer to any type of object and then used to access such an object or an array of such objects in the space allocated (until the space is explicitly deallocated).

It is not that explicit, but the paragraph mentions 'accessing an array of such objects', and in the C standard, arrays are:

An array type describes a contiguously allocated nonempty set of objects with a particular member object type, called the element type. (from §6.2.5)

Also note that subsequent calls to calloc, realloc, and malloc do not guarantee contiguity or ordering of memory (with other memory blocks already allocated).

This point is also specified in §7.20.3.

The order and contiguity of storage allocated by successive calls to the calloc, malloc, and realloc functions is unspecified.

Answer 4

Most likely it will not work. malloc can not rearrange already allocated memory so it can't make a continuous free block of 20 byte. It can only ask the OS to get another page of memory from, of which it can chop of 20 bytes + header.

Answer 5

OS specific.

If it's a system that has virtual memory, malloc is just going to allocate another page of VM and add it to its heap, and then hand you the memory you need. The chunk of memory within the page has to be contiguous though. Interestingly, if your demand is bigger than the page size, the memory doesn't have to be contiguous in physical memory. Two pages of VM can be made contiguous, regardless of where they are located in physical memory.

If it's a system that does not have VM, then yes, all the memory you request needs to be contiguous in physical memory.

Answer 6

The memory has to be continuous - what you are mentioning is generally referred to as memory fragmentation, and is a real problem for applications acquiring and freeing memory very often.